This guy starts to write down 21 x 13. He then draws 3 lines on a piece of paper horizontally, two lines for the number 2 and one line for the number 1. After he has done this, he then draws some vertical lines that symbolise the other numbers. This is so he can proceed to the next stage of the process, and this is the bit that will really blow your mind. He draws semi-circles around the corners of the design and he counts the amount of dots that there are in each section. After writing these down, he then puts all the numbers together and this is a great way to do math without having to get out a calculator.
This is one that you really do need to see for yourself because he does the same method with multiple sums, not to mention that the end result is always right on point.